Optical isotropization of anisotropic wurtzite Al-rich AlGaN via asymmetric modulation with ultrathin (GaN)m/(AlN)n superlattices

Abstract

973 program [2012CB619301, 2011CB925600]; "863" program [2011AA03A111]; National Natural Science Foundation of China [61106008, 11204254, 61204101, 90921002]; FRFCU [2012121011, 2011121042]; Natural Science Foundations of Fujian Province [2010J01343, 2012J01024]; fundamental research funds for the central universities [2011121042]Symmetric anisotropy in wurtzite semiconductors, e.g., AlGaN, has led to the significant optical anisotropy that is rather difficult to resolve. Here, a novel scheme for achieving optical isotropization in Al-rich AlGaN through the introduction of additional asymmetric elements is demonstrated to compensate the native asymmetry. Asymmetric modulation of alloy composition and periodicity of (GaN)m/(AlN)n superlatices was proposed with first-principles simulations. Results showed that the compensation for the c-axial symmetry with the asymmetric ultrathin (GaN)m/(AlN)n superlatices (m 2) could well achieve the equivalence of the ordinary and extraordinary imaginary dielectric functions epsilon 2 at the band edge. Measurement with spectroscopic ellipsometry for this (GaN)m/(AlN)n superlatice insertion in AlGaN host confirmed the theoretical predictions of the optical isotropization. This method can be transferred to other semiconductors in anisotropic structure and with troubles of optical anisotropy